- Chemical structure
- Other crystalline phases
- Applications
- How is it done?
- Physical and chemical properties
- References
The potassium nitrate is a metal alkali, and nitrate oxoanion potassium ternary compound salt. Its chemical formula is KNO 3, which means that for each ion K +, there is an ion NO 3 - interacting with this. Therefore, it is an ionic salt and constitutes one of the alkali nitrate (LiNO 3, NaNO 3, RBNO 3…).
KNO 3 is a strong oxidizing agent due to the presence of the nitrate anion. That is, it functions as a reserve for solid and anhydrous nitrate ions, unlike other highly water-soluble or highly hygroscopic salts. Many of the properties and uses of this compound are due to the nitrate anion, rather than the potassium cation.
KNO 3 crystals with needle shapes are illustrated in the image above. The natural source of KNO 3 is the saltpeter, known by the names Saltpeter or salpetre, in English. This element is also known as potash nitrate or nitro mineral.
It is found in arid or desert areas, as well as efflorescence from cavernous walls. Another important source of KNO 3 is guano, the excrement of animals that inhabit dry environments.
Chemical structure
In the upper image the crystal structure of KNO 3 is represented. The purple spheres correspond to the K + ions, while the red and blue are the oxygen and nitrogen atoms, respectively. The crystalline structure is of the orthorhombic type at room temperature.
The geometry of the anion NO 3 - is that of a trigonal plane, with the oxygen atoms at the vertices of the triangle, and the nitrogen atom at its center. It has one positive formal charge on the nitrogen atom, and two negative formal charges on two oxygen atoms (1-2 = (-1)).
These two negative charges of NO 3 - delocalize between the three oxygen atoms, always maintaining the positive charge on nitrogen. As a result of this, the ions K + in the glass avoid positioned just over or under nitrogen anions NO 3 -.
In fact, the image demonstrates how the K + ions are surrounded by the oxygen atoms, the red spheres. In conclusion, these interactions are responsible for the crystal arrangements.
Other crystalline phases
Variables such as pressure and temperature can modify these arrangements and originate different structural phases for KNO 3 (phases I, II and III). For example, phase II is that of the image, while phase I (with trigonal crystal structure) is formed when the crystals are heated up to 129 ºC.
Phase III is a transitional solid that is obtained from phase I cooling, and some studies have shown that it exhibits some important physical properties, such as ferroelectricity. In this phase the crystal forms layers of potassium and nitrates, possibly sensitive to electrostatic repulsions between the ions.
In the phase III layers, the NO 3 - anions lose a little of their planarity (the triangle bends slightly) to allow this arrangement, which, in the event of any mechanical disturbance, becomes the phase II structure.
Applications
Salt is of great importance since it is used in many human activities, which are manifested in industry, agriculture, food, etc. These uses include the following:
- The preservation of food, especially meat. Despite the suspicion that it is involved in the formation of nitrosamine (a carcinogenic agent), it is still used in delicatessens.
- Fertilizer, because potassium nitrate provides two of the three macronutrients in plants: nitrogen and potassium. Along with phosphorus, this element is necessary for the development of plants. That is, it is an important and manageable reserve of these nutrients.
- Accelerates combustion, being able to produce explosions if the combustible material is extensive or if it is finely divided (greater surface area, greater reactivity). In addition, it is one of the main components of gunpowder.
- Facilitates the removal of stumps from felled trees. Nitrate supplies the nitrogen needed for fungi to destroy stump wood.
- It intervenes in the reduction of dental sensitivity through its incorporation in toothpastes, which increases the protection to the painful sensations of the tooth produced by cold, heat, acid, sweets or contact.
- It intervenes as a hypotensive in the regulation of blood pressure in humans. This effect would be given or interrelated with a change in sodium excretion. The recommended dose in treatment is 40-80 mEq / day of potassium. In this regard, it is pointed out that potassium nitrate would have a diuretic action.
How is it done?
Most of the nitrate is produced in the mines of the deserts in Chile. It can be synthesized by various reactions:
NH 4 NO 3 (aq) + KOH (aq) => NH 3 (aq) + KNO 3 (aq) + H 2 O (l)
Potassium nitrate is also produced by neutralizing nitric acid with potassium hydroxide in a highly exothermic reaction.
KOH (aq) + HNO 3 (conc) => KNO 3 (aq) + H 2 O (l)
On an industrial scale, potassium nitrate is produced by a double displacement reaction.
NaNO 3 (aq) + KCl (aq) => NaCl (aq) + KNO 3 (aq)
The main source of KCl is from the mineral silvin, and not from other minerals such as carnalite or cainite, which are also composed of ionic magnesium.
Physical and chemical properties
Potassium nitrate in solid state appears as a white powder or in the form of crystals with an orthorhombic structure at room temperature, and trigonal at 129 ºC. It has a molecular weight of 101.1032 g / mol, is odorless, and has an acrid saline taste.
It is a very soluble compound in water (316-320 g / liter of water, at 20 ºC), due to its ionic nature and the ease that water molecules have to solvate the K + ion.
Its density is 2.1 g / cm 3 at 25 ºC. This means that it is approximately twice as dense as water.
Their melting points (334 ºC) and boiling points (400 ºC) are indicative of the ionic bonds between K + and NO 3 -. However, they are low compared to those of other salts, because the crystalline lattice energy is lower for monovalent ions (that is, with charges ± 1), and they also have not very similar sizes.
It decomposes at a temperature close to the boiling point (400 ºC) to produce potassium nitrite and molecular oxygen:
KNO 3 (s) => KNO 2 (s) + O 2 (g)
References
- Pubchem. (2018). Potassium Nitrate. Retrieved on April 12, 2018, from: pubchem.ncbi.nlm.nik.gov
- Anne Marie Helmenstine, Ph.D. (September 29, 2017). Saltpeter or Potassium Nitrate Facts. Retrieved on April 12, 2018, from: thoughtco.com
- K. Nimmo & BW Lucas. (May 22, 1972). Conformation and Orientation of NO3 in α-Phase Potassium Nitrate. Nature Physical Science 237, 61–63.
- Adam Rędzikowski. (April 8, 2017). Potassium nitrate crystals.. Retrieved on April 12, 2018, from:
- Acta Cryst. (2009). Growth and single-crystal refinement of phase-III potassium nitrate, KNO 3. B65, 659-663.
- Marni Wolfe. (October 03, 2017). Potassium Nitrate Risks. Retrieved on April 12, 2018, from: livestrong.com
- Amethyst Galleries, Inc. (1995-2014). The mineral niter. Retrieved on April 12, 2018, from: galleries.com